Abstract : Experiments in the transmission of radio waves through subsurface rock strata were conducted on Cape Cod, Massachusetts, in the Adirondack region of New York, and in the Cheyenne Mountain area of Colorado. Linear vertical antennas were placed in deep drill holes. The transmission path lengths were 1 mile on Cape Cod and 1 and 3 miles in the Adirondack region. At Cheyenne Mountain, where the up-over mode was emphasized, the path through the rock was approximately 700 feet. The subsurface media encountered in these areas were found to be quite inhomogeneous, and on the whole, to have relatively high values of conductivity, larger than 10 to the minus 4th power mhos/ meter. Several techniques were used to ascertain whether the transmissions were due to radio waves propagating directly through the rock strata or via the 'up-over-anddown' path. The bulk conductivities of the rock media along the transmission paths were deducted from measurements of the attenuation and the phase constants. The local conductivities in the vicinity of the drill holes were deduced from the measured impedance characteristics of the antennas. An improved antenna probe technique was used to determine the fine-grain variations of conductivity with depth in the drill holes. (Author)